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XTE J1810-189 underwent an outburst in 2008, and was observed over $\sim 100$d by RXTE. Performing a time-resolved spectral analysis on the photosphericradius expansion burst detected on 2008 May 4, we obtain the source distance inthe range of 3.5--8.7 kpc for the first time. During its outburst, XTEJ1810-189 did not enter into the high/soft state, and both the soft and hardcolours decreased with decreasing flux. The fractional rms remained at highvalues ($\sim 30$ per cent). The RXTE/PCA spectra for 3-25 keV can be describedby an absorbed power-law component with an additional Gaussian component, andthe derived photon index $\Gamma$ increased from $1.84\pm0.01$ to $2.25\pm0.04$when the unabsorbed X-ray luminosity in 3-25 keV dropped from $4\times10^{36}$ergs s$^{-1}$ to $6\times10^{35}$ ergs s$^{-1}$. The relatively high flux,dense observations and broadband spectra allow us to provide strong evidencethat the softening behaviour detected in the outburst of XTE J1810-189originates from the evolution of non-thermal component rather than the thermalcomponent (i.e. neutron star surface emission).

X-ray softening during the 2008 outburst of XTE J1810-189